Scalability and load balancing in network servers are issues which have received considerable attention in light of the expansion of the Internet. For example, it may be desirable to have multiple servers servicing customers. The workload of such servers may be balanced by providing a single network visible Internet Protocol (IP) address which is mapped to multiple servers.
Such a mapping process may be achieved by, for example, network address translation (NAT) facilities, dispatcher systems and Dynamic Name Server/WorkLoad Management (DNS/WLM) systems from International Business Machines Corporation (IBM), Armonk, N.Y. These various mechanisms for allowing multiple servers to share a single IP address are illustrated in FIGS. 1 through 3. FIG. 1 illustrates a conventional network address translation system as described above. In the system of FIG. 1, a client 10 communicates over a network 12 to a network address translation (NAT) system 14. The network address translation system receives the communications from the client 10 and converts the communications from the addressing scheme of the network 12 to the addressing scheme of the network 12′ and sends the messages to the servers 16. A server 16 may be selected from multiple servers 16 at connect time and may be on any host, one or more hops away. All inbound and outbound traffic flows through the NAT system 14.
FIG. 2 illustrates a conventional DNS/WLM system as described above. The server 16 is selected at name resolution time when the client 10 resolves the name for the destination server from the DNS/WLM system 17 which is connected to the servers 16 through the coupling facility (CF) 19. The DNS/WLM system 17 of FIG. 2 relies on the client 10 adhering to a “zero time to live” lifetime for IP addresses such that IP addresses are not cached by the client.
FIG. 3 illustrates a conventional dispatcher system. As seen in FIG. 3, the client 10 communicates over the network 12 with a dispatcher system 18 to establish a connection. The dispatcher routes inbound packets to the servers 16 and outbound packets are sent over network 12′ but may flow over any available path to the client 10. The servers 16 are typically on a directly connected network to the dispatcher 18 and a server 16 is selected at connect time.
Such a dispatcher system is illustrated by the Interactive Network Dispatcher function of the IBM 2216 and AIX platforms. In these systems, the same IP address that the Network Dispatcher node 18 advertises to the routing network 12 is activated on server nodes 16 as loopback addresses. The node performing the distribution function connects to the endpoint stack via a single hop connection because normal routing protocols typically cannot be used to get a connection request from the endpoint to the distributing node if the endpoint uses the same IP address as the distributing node advertises. Network Dispatcher uses an application on the server to query a workload management function (such as WLM of System/390), and collects this information at intervals, e.g. 30 seconds or so. Applications running on the Network Dispatcher node 18 can also issue “null” queries to selected application server instances as a means of determining server instance health.
In addition to the above described systems, Cisco Systems offers a Multi-Node Load Balancing function on certain of its routers that performs the distribution function. Such operations appear similar to those of the IBM 2216.
Finally, in addition to the system described above, AceDirector from Alteon provides a virtual IP address and performs network address translation to a real address of a selected server application. AceDirector appears to observe connection request turnaround times and rejection as a mechanism for determining server load capabilities.